1. Field of the Invention
The present invention relates to devices for producing steam from liquid water, the steam being used more particularly in food cooking apparatus.
2. Description of the Prior Art
Numerous steam generators are known which generally comprise a generator enclosure adapted for containing water, means for feeding water into the enclosure, a steam conduction duct for feeding the steam into a user enclosure and means for heating the water contained in the generator enclosure for bringing the water to boiling point and producing steam.
Thus, the document EP-A-0 323 939 describes such a steam generator comprising a horizontal lower duct, a horizontal upper duct, the two ducts being connected together by one or more parallel vertical heating tubes and by a return tube which is also vertical, a tapping of the return tube being fitted with water level detection means. The water contained in the generator can flow through a lower lateral orifice, closable by means of a valve, for emptying the generator at the end of use.
The Applicants have discovered a tendency to localized heating in such a steam generator, causing for example excessive heating of the heating tubes whereas the other tubes operate correctly at appropriate temperatures.
It appears that the heating problems do not occur at the beginning of operation but occur only after a sufficient operating time.
A thorough study of the problem has shown that the localized heating may be overcome by correctly removing the scale waste which tends to accumulate in the lower part of the steam generator.
Thus, the problem proposed by the invention is to suppress simply, automatically and inexpensively the excessive differential heating appearing on the heating elements of steam generators after an operating time.
The solution must allow the scale appearing at the low part of the steam generator to be sufficiently removed.
A first solution is suggested in the document EP-A-317 444 which describes a steam generator in which the heating means are of a special kind, using porous bodies. The porous heating body is plunged into an enclosure containing water and defined by a lower wall having an orifice discharging into a lower discharge duct closed by a valve. The document suggests removing the particles of scale by opening the lower valve for the flow of water, and simultaneously feeding rinsing water through an upper duct. Such a cleaning procedure requires a complementary volume of rinsing water and means for feeding and controlling the rinsing water flow, which substantially increases the cost of the installation. In addition, such a structure, with a water feed at the upper part of the enclosure would be difficult to apply to a steam generator in which the heating means are vertical tubes, for it would then be necessary to introduce the rinsing water into each of the tubes separately.
Another solution is proposed in the documents DE-U-8 901 904 and EP-A-383 327. In these documents, the heating means are introduced into an enclosure filled with water to an intermediate level. A suction and delivery pump causes the discharge water to flow into an intermediate duct disposed between the pump and the lower wall of the enclosure of the generator. These documents recommend placing the water discharge orifice in the lower enclosure wall close to the lateral wall of the enclosure, and positioning a rinsing water injection nozzle in a diametrically opposite situation. The combination of the pump and the rinsing nozzle produces a violent fluid stream in the tank, for cleaning it. This structure is also complex and expensive, since it requires not only a complementary supply of rinsing water, but also means for feeding the rinsing water, for controlling the rinsing water and a discharge pump. Furthermore, this solution would not be adaptable to cleaning steam generators in which the heating means are vertical tubes: the means described do not allow a fluid stream to be created inside the tubes, since the stream produced is perpendicular to the axis of the heating tubes.